This invention relates generally to magnetic resonance imaging (MRI), and more particularly, the invention relates to steady-state magnetic resonance imaging sequences and a method of establishing nuclear magnetization in a steady-state position to thereby accelerate MRI signal acquisition.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Magnetic resonance (MR) imaging is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During a MR experiment, each nuclear spin responds to four different effectsxe2x80x94precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MR experiments, a combination of these effects occurs periodically.
Refocused steady-state free precession (SSFP) sequences have recently gained popularity in magnetic resonance imaging, due to improved gradient hardware. One drawback with such sequences is the slow and often oscillatory signal progression as a steady-state is established, as shown in FIG. 1a. Imaging during this transient period can result in image artifacts. Alternatively, waiting for a steady-state reduces the scan-time efficiency of the method. An objective of the present invention is to xe2x80x9ccatalyze xe2x80x9d or speed up the evolution of a steady-state as in FIG. 1b. An ideal catalyzing sequence would achieve the steady-state much more quickly and with no oscillation.
The steady-state nuclear magnetization in steady-state sequences is a non-trivial function of many parameters. The present invention is directed to generating a sequence that catalyzes the steady-state magnetization based on the steady-state and the transient response in MRI sequences.
In accordance with the invention, a steady-state condition for tipped nuclear spins is accelerated or catalyzed by first determining magnetization magnitude of the steady-state and then scaling magnetization along one axis (Mz) to at least approximate the determined magnetization magnitude. Then the scaled magnetization is rotated to substantially coincide with a real-valued eigenvector extension of the tipped steady-state magnetization. Any error vector will then decay to the steady-state condition with reduced oscillation.
In one embodiment, the magnetic resonance imaging utilizes steady-state free precession (SSFP). The scaling and rotating steps are followed by the steps of applying read-out magnetic gradients and detecting magnetic resonance signals from the tipped nuclear spins. The magnetization magnitude is determined by eigenvector analysis, and the eigenvector extension is a real-valued eigenvector determined in the analysis.